U.S. patent number 6,118,503 [Application Number 09/399,124] was granted by the patent office on 2000-09-12 for light guide device enhancing a polarized component and liquid crystal display device.
This patent grant is currently assigned to IBM, Stanley Electric Co., Ltd.. Invention is credited to Koji Kawada, Yoji Oki, Masaru Suzuki.
United States Patent |
6,118,503 |
Oki , et al. |
September 12, 2000 |
Light guide device enhancing a polarized component and liquid
crystal display device
Abstract
A polarized component is obtained with a high conversion
efficiency in a light guide which produces one of the polarized
components by having it transmitted. The light from a light source
is incident to a light guide which comprises a plurality of light
guide layers and reflected by the end surface to an interface
between the light guide layers. The polarized component
transmitting through the end surface is rotated in its polarization
plane by a wave length plate and reflected by a reflecting plate
for reentrance to the light guide at the end surface of the light
guide toward the interface. The reentering light mostly transmits
through the interface because the polarization plane is rotated. A
reflected light polarized component is returned to the wave length
plate and the reflecting plate, and directed back to the interface
again. The polarized component transmitting through the interface
is similarly transmitted and reflected in the next interface. The
number of interfaces can be reduced by increasing the reflection of
the polarized component reflected in the interface. For this
purpose, the index of refraction in the direction along the axis of
the reflected polarized component is increased by making the index
of refraction of the light guide layer anisotropic.
Inventors: |
Oki; Yoji (Kanagawa,
JP), Kawada; Koji (Kanagawa, JP), Suzuki;
Masaru (Kanagawa, JP) |
Assignee: |
Stanley Electric Co., Ltd.
(Tokyo, JP)
IBM (Armonk, NY)
|
Family
ID: |
17462221 |
Appl.
No.: |
09/399,124 |
Filed: |
September 20, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Sep 22, 1998 [JP] |
|
|
10-268704 |
|
Current U.S.
Class: |
349/65; 349/66;
362/616 |
Current CPC
Class: |
G02B
6/003 (20130101); G02B 6/0038 (20130101); G02B
6/0046 (20130101); G02B 6/005 (20130101); G02B
6/0055 (20130101); G02B 6/0056 (20130101); G02B
6/0053 (20130101); G02F 1/13362 (20130101); G02F
1/133615 (20130101) |
Current International
Class: |
F21V
8/00 (20060101); G02F 1/13 (20060101); G02F
1/1335 (20060101); G02F 001/1335 () |
Field of
Search: |
;349/95,62,65,66
;385/129 ;362/31 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
5845035 |
January 1998 |
Wimberger-Friedl |
|
Foreign Patent Documents
Primary Examiner: Dudek; James A.
Attorney, Agent or Firm: Hancock; Earl C. Holland & Hart
LLP
Claims
What is claimed is:
1. A light guide device comprising:
a light guide unit consisting of a lamination of a plurality of
light guide layers in which one end thereof is a light incidence
surface and the other end is cut obliquely with respect to the
direction of the lamination;
a reflecting plate disposed adjacent to said other end surface;
and
means disposed between said other end surface and said reflecting
plate for changing the polarization direction of light,
said light guide layers having an anisotropic index of refraction
in which the index of refraction in the axis of reflecting one of
the polarized components of the light from said incident surface is
larger than the index of refraction in the axis of transmitting the
other one of said polarized components.
2. A light guide device of claim 1 in which said other end surface
is cut obliquely in an angle such that the light reflected by said
other end surface and incident to an interface between said light
guide layers is incident in the Brewster angle.
3. A light guide device of claim 1 in which said other end surface
has a plurality of sloped surfaces in the direction perpendicular
to the direction of the lamination of the light guide layers and
the direction of incidence of the light.
4. A light guide device of claim 3 in which said sloped surface
comprises a combination of a surface having an angle of inclination
smaller than the angle of inclination of said other end surface and
a surface having a larger angle of inclination which is an angle
causing the light reflected from the latter surface and incident to
the interface between the light guide layers to be incident in the
Brewster angle.
5. A light guide device of claim 4 in which the angle .phi. of the
surface having a larger angle is given by the following
expression:
where n.sub.1 is an index of refraction of the light guide and
n.sub.2 is an index of refraction of a material other than the
light guide.
6. A light guide device of claim 5 in which a plurality of light
guide films are further laminated on the top layer of said
plurality of light guide layers of said light guide unit.
7. A light guide device of claim 6 in which said light guide film
is thinner than said light guide layer.
8. A light guide device of claim 6 in which a phase film
for changing the direction of polarization is further disposed on
said plurality of light guide films.
9. A liquid crystal display device comprising a liquid crystal cell
and a light guide device disposed in the back of said liquid
crystal cell, said light guide device comprising:
a light guide unit consisting of a lamination of a plurality of
light guide layers in which one end thereof is a light incidence
surface and the other end is cut obliquely with respect to the
direction of the lamination;
a reflecting plate disposed adjacent to said other end surface;
means disposed between said other end surface and said reflecting
plate for changing the polarization direction of a light; and
a prism sheet disposed on the top light guide layer of said light
guide unit and having an apex part only on one side thereof, the
side where said apex is formed facing said light guide unit,
said light guide layers having an anisotropic index of refraction
in which the index of refraction in the axis of reflecting one of
the polarized components of the light from said incident surface is
larger than the index of refraction in the axis of transmitting the
other one of said polarized components.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light guide unit for use in a
liquid crystal display device in which a polarized component of
light is enhanced and a liquid crystal display device which is
provided with such light guide unit. Particularly, this invention
relates to a light guide unit for efficiently converting the light
from a light source to a polarized light and a liquid crystal
display device having means for efficiently directing the polarized
light emitted from such light guide unit to a liquid crystal
cell.
2. Description of the Related Art
A liquid crystal display device is conventionally observed by
directing polarized light to a liquid crystal cell to cause the
polarization plane to be rotated depending on the condition of the
cell for passage through a polarizer plate. A light source of the
polarized light is placed in the back of the liquid crystal plate
and thus is called a "back light". For obtaining such polarized
light wave, a non-polarized light was conventionally incident to a
polarizer plate and either one of the polarized components; i.e., S
component and P component, was absorbed.
Assuming that a plane defined by a light incident to a point of
incidence on a surface is an incident plane, a polarized component
parallel to the incident plane is called a P component while a
component perpendicular to the incident plane is called an S
component. Therefore, more than 50-percent of an incident light was
not effectively utilized in principle and an actual measurement
shows that about 58-percent of the incident light is absorbed.
Further, a light dispersing sheet having printed dots was typically
used in addition to a polarization device for obtaining polarized
light by absorbing a polarized component in a conventional Liquid
Crystal Display (LCD) device, and this makes an additional
20-percent of the light unavailable.
In FIG. 1, a LCD module 100 of a conventional LCD device is shown.
The light emanating from a light source 101 transmits through a
light guide plate 102 having 96% transmittance, a dispersion sheet
103 having 80% transmittance, a lower polarizer plate 104 having
42% transmittance, a glass substrate 105 having a numerical
aperture of 40%, a color filter 106 having 30% transmittance, and
an upper polarizer plate 107 having 90% transmittance, resulting in
an actually available light intensity which is 3.5% of the light
generated in the light source 101. This greatly prevents the energy
from being utilized efficiently.
A back light system of a high intensity for use in a low power
consumption LCD device is especially desired because it is an
important objective in a portable personal computer to assure a
longer usable time with a given capacity of a battery and the power
consumption of a back light 108 is a major percentage of total
power consumption.
Also, the light energy absorbed in the lower polarizer plate 104,
etc., is converted to heat energy which contributes to degradation
of parts of the LCD device. Particularly for a liquid crystal
material of STN (Super Twisted Nematic) type in which the display
quality is degraded by heat, it is an important objective to reduce
such heat generation. As seen from FIG. 1, 66.4% of the light
energy is converted to heat energy by the light absorption in the
lower polarizer plate 104 and the dispersion sheet 103 (this is 69%
of heat generation by the light energy).
In order to solve such problems, the applicant of this application
filed Japanese patent application no. 9-249139 relating to a method
of improving the efficiency of light utilization in obtaining a
polarized light by making available for use at least a part of a
polarized component which had not been utilized. The principle of
this method is shown in FIG. 3.
Light from a fluorescent lump CFL which is a light source is
incident to the end surface of a laminated light guide plate unit
via a reflecting mirror and a collimator. It propagates through the
layers of the light guide plates, and arrives at the other end
surface which is cut in an angle. The incident light is partly
reflected at the other end surface with the rest being transmitted
therethrough. The polarization plane of the light transmitting
through the end surface is rotated by a quarter wave length plate
placed thereunder and reflected by a reflecting plate placed under
the quarter wave length plate for reentrance to layers of the light
guide plate again through the quarter wave length plate as a P
component.
The P component reentering the light guide plates is incident to
the interface with an adjacent light guide plate layer. The angle
of incidence of the light on the interface is the Brewster angle
(to be described later in detail). Therefore, all the P component
and a part of the S component of the light incident to the
interface transmit through the interface with the rest of the S
component reflected back to the quarter wave length plate and the
reflecting plate. The light reflected again by the reflecting plate
is again directed to the interface after being converted to a P
component by the quarter wave length plate where all the P
component and a part of the S component, if any, transmit with the
rest being reflected.
The light reflected here is reflected repeatedly in a similar
manner and a light converted to a P component for each reflection
transmits through the interface. As such, the light guide unit
ultimately emits a large portion of the light from the light source
as a P component. The polarized light is emitted in the direction
largely deviated from the normal to the front. A prism sheet for
redirecting the light to the front toward the liquid crystal cell
is used. The polarization can be further improved by placing a
further polarization plate on the prism sheet.
Because the reflectance and transmission characteristics are
different between the S component and the P component, the light
transmitting through the interface and the light reflected by the
interface have different polarization components. To explain the
principle of operation of this invention, a change of polarization
components of the light in transmitting through or reflecting from
the interface between materials of different indices of refraction
is described with reference to FIGS. 4, 5 and 6.
In FIG. 4, when light 204 reaches an interface 203 between two
materials 201 and 202 having different indices of refraction
n.sub.1 and n.sub.2, respectively, a part of the light 205 is
reflected when the angle of incidence .phi..sub.1 is less than a
critical angle while a part of the light 206 transmits through the
interface. Assuming that a plane defined by a light incident to a
point of incidence on a surface is an incident plane, the incident
light 204 is divided into a P component parallel to the incident
plane and an S component perpendicular to the incident plane.
Modifying Maxwell equation for a dielectric material, the
transmittance of the polarized components P and S are given by;
where
Tp: transmittance of P component (1-reflectance Rp)
Ts: transmittance of S component (1-reflectance Rs)
.phi..sub.1 : incident angle of light
.phi..sub.2 : exit angle of light
n.sub.1 : index of refraction of material 201
n.sub.2 : index of refraction of material 202
or it is known that;
where
Rp reflectance of P component (1-transmittance Tp)
Ts: reflectance of S component (1-transmittance Ts)
The reflectance of the P polarized component and S polarized
component vary depending on the incident angle .phi..sub.1 and the
exit angle .phi..sub.2 as shown in FIG. 5 and FIG. 6, and differ
from each other even in a same incident angle .phi..sub.1
(reflectance/transmittance characteristics are different between S
and P polarized components).
For example, when the light proceeds from an acrylic material
having an index of refraction of 1.49 to air which has an index of
refraction of 1.00 (FIG. 6), the critical angle in which a total
reflection takes place is 42.1-degrees. If the light is incident at
40-degrees which is less than the critical angle, the exit angle
.phi..sub.2 will be 77.8-degrees according to Snell's law.
Substituting the above equation of Rs and Rp with this, the
reflectance for the S component is 35.69% while the reflectance for
the P component is 7.98%.
It should be clearly understood from the above description
referring to FIGS. 4 to 6 how the polarized components of the light
are transmitted and reflected in the interface in this
invention.
It is understood from the above-described principle that it is
important for the layers of the light guide to be laminated in
multiple layers to cause the unnecessary S component to be
reflected back each time the light reaches the interface between
the layers and to be returned as a P component for transmitting
through the interface thereby improving the efficiency of
converting the light emitting from the unit eventually to a P
component.
However, it is disadvantageous to laminate too many layers from the
view point of the efficiency of utilizing the energy of the light
source because each layer invites some loss of light. In addition,
the increased number of laminated layers would result in the
increase of the thickness of the entire unit even if a thin layer
is used. The increase of the thickness would also invite an
increase of the weight. It is the most important objective for a
portable information processing device, such as a notebook
computer, to decrease the power consumption of its battery as well
as the thickness and the weight of the entire unit as much as
possible.
SUMMARY OF THE INVENTION
This invention relates to an improvement of a light guide unit of
the above-described type, and it is an object of this invention to
provide a light guide unit having an unchanged performance with a
decreased thickness of the entire unit.
It is another object of this invention to improve the brightness of
a liquid crystal display device without resulting in an increase of
power consumption by efficiently combining the polarized light from
such light guide unit of a high efficiency to a liquid crystal
cell.
The basic configuration of this invention lies in a structure in
which the light from a light source incident to an end surface of a
unit of laminated light guide plates propagates through each layer
of the light guide plates and is partly reflected by the other end
surface which is obliquely cut. The rest of the light transmitting
therethrough causes the polarization plane of the transmitting
light to be rotated by a wave length plate lying thereunder and
reflected by a reflecting plate lying under the wave length plate
for reentrance to the light guide plate again through the wave
length plate as a P component.
The P component reentering the light guide plate is incident to an
interface between neighboring light guide plates. The incident
angle of the light incident to the interface is adapted to be the
Brewster angle. Therefore, all P component light incident to the
interface and a part of the S component light transmit the
interface while the rest of the S component light is reflected back
to the wave length plate and the reflecting plate. The light
reflected again by the reflecting plate is directed back to the
interface after being converted to a P component by the wave length
plate, and all P components and a part of S component, if any,
transmit through the interface while the rest is reflected.
The light reflected here is subject to the same process repeatedly,
and a light converted to a P component in every repetition
transmits through the interface. As such, the light guide unit
eventually emits a large portion of the light from the light source
as a P component. Because the polarized light is emitted in the
direction largely deviated from the normal to the front, a prism
sheet for redirecting the light to the front toward the liquid
crystal cell is used.
In this invention, it is important in the principle of this
invention that the S component is reflected in the interface of the
light guide layers. The number of the interfaces; i.e., the number
of the light guide layers can be reduced by causing as much S
component as possible to be reflected to reduce the S component
transmitting through the interface.
This invention provides a conversion efficiency comparable to light
guide layers using an isotropic material with a lesser number of
light guide layers by using a material of an anisotropic index of
refraction as the light guide layers to improve the reflectance of
the S component in the interface. The axes of two indices of
refraction of the anisotropic material coincide with the planes of
P and S components, respectively. While the index of refraction in
the direction of the axis lying in the plane of the P component may
be the same as a conventional one, the index of refraction in the
plane of the S component is higher than the conventional one. The
higher, the better. It is seen from the expression of the
reflectance Rs described above that the reflectance of the S
component becomes larger when the index of refraction in the axis
of the plane of the S component in the interface is larger.
The light guide unit comprising laminated light guide layers of
such anisotropic index of refraction receives an incident light
from a light source at the end surface thereof which is a
cross-section of the laminated layers to cause a part of the
incident light to be reflected at the opposite end surface which is
obliquely cut and the rest of the light to be transmitted
therethrough. A quarter wave length plate is attached to the
obliquely cut end surface and a reflecting plate is provided under
the wave length plate.
The light transmitting through the end surface is reflected by the
reflecting plate after being rotated by the wave length plate and
is incident to the end surface after being rotated by the wave
length plate again. The light incident to the end surface is
incident to the interface where it is transmitted and reflected as
described herein. However, the majority of the S component is
reflected in the interface with the rest transmitting through the
interface in this invention. Therefore, the light from the light
source can be converted to the P component with a lesser number of
layers.
In this invention, it is preferred that the light incident to the
first interface of the light guide is incident in the Brewster
angle. It is readily seen by drawing a geometrical drawing that the
angle of incidence of the light to the obliquely cut end surface of
the light guide unit decides the angle of incidence at the
interface. In this invention, the angle of incidence of the light
to the obliquely cut end surface of the light guide unit is so
adjusted that the light incident to the first interface of the
light guide is incident in the Brewster angle.
The light guide unit is so inclined with respect to the wave length
plate and the reflecting plate as to provide an incident angle
decided in this manner. In order to reduce the inclination, a
plurality of slopes making such incident angle may be formed in the
obliquely cut end surface. This allows a necessary incident angle
to be provided without inclining the entire light guide unit in
this angle. This allows the thickness of the entire unit to be
further reduced.
In this invention, the light guide unit may be formed into a shape
of a triangular wedge consisting of the top layer of the laminated
layers, the obliquely cut end surface and the surface to which the
light from the light source is incident. This allows a wedge-shaped
space to be provided under the unit for receiving various
components. This is advantageous for a portable data processing
device in which a thin and light weight type is especially
desired.
In another aspect of this invention, the prism means for directing
the polarized light to the front has a plurality of prisms disposed
in a same pitch as columns of the liquid crystal cells. Each prism
has an incident surface and a reflecting surface. Because the light
is emitted from the reflecting surface, a portion corresponding to
the incident surface is dark. In this invention, the dark incident
surface portion is so disposed as not to contribute illuminating
the liquid crystal cell by having the portion corresponding to the
reflecting surface align the columns of the liquid crystal cell.
All the polarized light emitted from the light guide is thus
directed to the liquid crystal cell.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing a conventional LCD
device.
FIG. 2 is a diagram showing the structure of a conventional LCD
polarizer plate unit.
FIG. 3A is a diagram showing the structure of a conventional LCD
polarizer plate unit.
FIG. 3B is an expanded view of a segment of FIG. 3A.
FIG. 4 is a diagram showing refraction of light between different
materials.
FIG. 5 shows a characteristic plot of reflectance when the light is
incident from a material having an index of refraction of 1.0 to a
material having an index of refraction of 1.49.
FIG. 6 shows a characteristic plot of reflectance when the light is
incident from a material having an index of refraction of 1.49 to a
material having an index of refraction of 1.0.
FIG. 7 is a diagram showing deflection of the light by a prism
sheet.
FIG. 8 is a diagram showing deflection of the light by a prism
sheet.
FIG. 9 is a schematic diagram showing a concept of another
embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The basic structure of this invention is shown in FIG. 2. The
laminated light guide unit is made of thin light guide layers
laminated as shown and a light source is attached to an end surface
thereof The light source comprises a fluorescent lamp and a
reflecting sheet. The lamination is cut so as to assume an oblique
end surface to which a combination of a quarter wave length plate
and a reflecting plate is affixed.
The end surface makes an angle .phi. with respect to the quarter
wave length plate and the reflecting plate. Therefore, the light is
incident to the end surface at an angle of .pi./2-2.phi.. It is
readily found from a geometrical drawing that an incident angle to
the interface between layers is .pi./2-2.phi..
It is preferable that this incident angle is the Brewster angle.
When the light is incident to the interface at the Brewster angle,
all polarization component lying in the incident plane (P)
transmits the interface while all polarization component lying in a
plane orthogonal to the incident plane (S) is reflected. Any S
component which may transmit through the interface will be
reflected by the next interface. The S component reflected from the
interface is returned back to the light guide layers by the wave
length plate and the reflecting plate as a P component and incident
again to the interface. The S component is reflected each time the
above process is repeated at a plurality of interfaces transmits
through the interface as a P component so that a large portion of
the light from the light source is emitted from the light guide
unit as a P component.
A plurality of thin light guide films may be further laminated on
the top light guide layer of the light guide unit as shown in FIG.
3. This further adds interfaces of transmission and reflection.
The light guide member and the plurality of light guide layers are
preferably of a material which assumes a low internal absorption of
the light, such as an acrylic sheet and preferably transparent
materials including acrylic resin, PMMA (polymethylmethacrylate),
polycarbonate, polyethylene, Se, and AgCl. The shape of the light
guide member may be in a shape suitable for use such as a bar and a
curved surface without being limited to a plate and a sheet.
The light guide member may be of a single piece or a lamination of
a plurality of sheets. These light guides are not limited to a same
size or a same material and a member requiring stiffness may be
designed thick while a member which does not require stiffness may
be designed thin. Also, materials of differing indices of
refraction may be deposited in multiple layers on a stiff light
guide to increase the number of laminated layers while maintaining
a stiffness.
In using an acrylic sheet in the light guide member, the thickness
of the sheet is preferably 0.1 to 4.0 mm from the consideration of
the stiffness and the efficiency of light utilization. The
lamination as used in this invention is not limited to insertion of
air between the light guides and water vapor may be introduced
between the guides for preventing degradation of the light guide
unit, water or an adhesive may be inserted between the guides for
preventing the guides from being peeled off, or a material having
an index of refraction differing from the light guide may be
inserted. Higher reflectance of the reflecting plate is preferable
in this invention and the reflecting plate may be made of an
aluminum deposited sheet, a silver deposited sheet and a metal
foil, etc.
In this invention, the light guide layer is made of a material
having a high index of refraction in the axis lying in the plane of
the S component. For example, while the isotropic index of
refraction of an acrylic material is normally 1.49, the index can
be increased up to about 1.69 in the direction of the axis lying in
the plane of the S component. By doing so, an increased portion of
the S component is reflected in the interface (lesser amount of the
S component transmit the interface) so that an unchanged effect can
be resulted with a lesser number of layers than those required for
an isotropic material.
For example, when an acrylic material having an index of refraction
1.49 is used as an isotropic material, the reflectance of the S
component is 28% while the transmittance is 72% per layer. With ten
layers laminated, the overall transmittance will be 0.72.sup.10
=0.04.
On the other hand, if the index of refraction in the direction of
the S component is 1.66, the reflectance is 40% while the
transmittance is 60% and the same effect is obtained with 6 layers
(0.6.sup.6 =0.04). A sheet having such anisotropic index of
refraction is easily available in the market.
While the thickness of the light guide film is not important and it
is preferable that the number of the interfaces is as large as
possible, the light guiding layer is preferably as thin as possible
from the view point of reducing the weight of the light guide unit.
An extra space is created by making the thickness of the light
guiding layer in this portion extremely thin and the layers of
substantially same size may be used in lamination without requiring
the layers to be progressively in different sizes resulting in a
stepped structure as shown in FIG. 2.
As such, the light is not lost by re-entering from the edge of the
layers and dark and bright stripes are eliminated. Even if the
steps remain in the layers as shown in FIG. 2, there is little
possibility of the light re-entering and recognizable stripes are
not generated because the layer is thin and the size of the edge is
very small.
By employing the above structure, this invention allows the
cross-sectional shape of the light guide unit to be of a triangular
shape as shown in FIG. 3, in contrast to the conventional unit
which had a rectangular cross-section as shown in FIG. 2. By this
structure, the weight and the
volume of the unit can be about half the conventional unit. Also,
this invention can implement a mode which is similar to the mode in
which a conventional back light (not generating a polarized light)
uses a light guide of a wedge shaped cross section to provide an
effective use of a space and allows a conventional back light to be
replaced with the present polarized back light in a form compatible
with the conventional type.
While the light guide layer is acrylic material and the surrounding
material is air in the example so far described, any material of
the layer and any surrounding material may be used so long as the
indices of refraction of the materials allow the incident light to
satisfy the Brewster angle or an angle which is near the Brewster
angle.
The following condition is required for the incident angle
.pi./2-2.phi. to be the Brewster angle .theta..sub.B. In the
expression, n.sub.1 is an index of refraction of the light guide,
n.sub.2 is an index of refraction of a material other than the
light guide (air in FIG. 5), and .phi. is the angle of the groove
(the slope of the larger angle of inclination). The relationship
between Brewster angle .theta..sub.B and n.sub.1, n.sub.2 is given
by;
The angle of incidence to the upper surface of the light guide is
given by a geometric analysis using .phi.;
Snell's law is expressed on the upper surface of the light guide
as;
solving this expression for .phi. gives the following general
solution;
Any medium satisfies the condition of this invention so long as it
satisfies the above general expression.
While the entire light guide unit is inclined with respect to the
wave length plate and the reflecting plate so as to provide an
incident angle which is equal to the Brewster angle, many sloped
surfaces which provide such incident angle can be formed in the
obliquely cut end surface. As shown in the enlarged view in FIG. 3,
many sloped surfaces running perpendicularly to the face of the
drawing are formed in the obliquely cut end surface and are so
disposed as to provide a desired angle to the incident light in the
light guide. An incident angle satisfying the Brewster angle is
thus provided though the entire light guide is not inclined in this
angle. A necessary incident angle can be thus provided while the
light guide unit is not entirely inclined in this angle thereby
reducing the thickness of the entire unit.
This invention is contemplated for use as a back light of a liquid
crystal display device. The liquid crystal display device comprises
a light source and glass substrates sandwiching a liquid crystal to
which a polarized light emitted from the light guide unit of this
invention is incident.
The light emitted from the light guide is largely inclined in
70-degrees from the front thereof in this invention. Two methods
are available for deflecting the light to the right angle to the
front surface. The first method is to have the light refract twice
to deflect it to the front, in which a prism sheet is used with the
apex thereof oriented upward as shown in FIG. 8. When the index of
refraction n of the material of the prism is 1.58, a prism sheet
having an angle of apex of 32-degrees is required to deflect the
light to the front.
A second method is to have the light refract once and totally
reflect once to deflect to the front, in which the prism sheet is
used with the apex thereof oriented downward as shown in FIG. 9. In
this case, a prism having an angle of apex of 65.4-degrees is
required. As seen in the above, a same effect is resulted whether
the prism is oriented upward or downward. From the view point of
fabrication, it is more advantageous in the view point of yield and
cost to use the prism with the apex oriented downward because a
smaller apex angle of a prism is more difficult to fabricate (a
larger apex angle can be used when the apex is oriented downward).
The prism sheet is made of a glass or plastic material.
In FIGS. 7 and 8, it is seen that the sloped surface of each prism
which is not the light reflecting surface does not emit the light
to the front. In other words, the light emitted from the prism
sheet is in a stripe pattern. This may possibly induce an
interference pattern with a gate line or a data line of the liquid
crystal cell. In order to prevent the stripe pattern from being
generated, the pitch of the prisms of the prism sheet (50 microns,
for example) can be made smaller than the pitch of the liquid
crystal cell (200 microns, for example) to mismatch the pitches. By
doing so, the prism sheet is observed as if it emits the light
uniformly from the front and the interference pattern can be
prevented from being generated because the pitch of the prism sheet
is very small.
However, the light incident to a portion of the liquid crystal cell
array which has no opening is absorbed there and wasted in this
case. Another aspect of this invention provides a structure in
which such waste is avoided.
According to this structure of this invention, the pitch of the
prisms of the prism sheet is made the same as the pitch of the
liquid crystal cell array so that the opening part of the liquid
crystal cell coincides with a portion of the prism corresponding to
the reflecting surface which emits the light. The portion
corresponding to the slope of the prism which is not the reflecting
surface coincides with the part having no opening.
FIG. 9 is a schematic diagram showing a concept of the inventive
structure. As shown in FIG. 9, the pitch of the prisms of the prism
sheet is made the same as the pitch of the liquid crystal cell
array. The light reflected by the reflecting surface of the prism
is directed to the opening part of the liquid crystal cell. The
part having no opening does not receive the light because it faces
to the surface which is not a reflecting surface. All the light
emitted from the light guide unit is thus directed to the opening
part, and there is no light which is absorbed without being
utilized. It is easy to manufacture the prism because the prism has
a larger pitch than those shown in FIGS. 7 and 8. The apex angle
and the ratio of reflecting/transmitting surfaces may be suitably
decided in a specific design work.
As shown in FIG. 9, the liquid crystal cell array may be formed
directly on the prism sheet. In this case, the prism sheet also
plays a role of a glass substrate of the liquid crystal cell. The
number of interfaces between media is decreased by 2 when compared
to a case where an independent prism sheet is disposed between the
liquid crystal cell and the light guide film, resulting in a
corresponding improvement of efficiency.
The thickness and the weight of the light guide are reduced because
the light is converted to the P polarized component with the number
of layers less than those of a conventional light guide according
to this invention. There is no light which is absorbed without
being utilized in another aspect of this invention because all the
light from the light guide is directed to the opening part of the
liquid crystal cell.
The following is a brief description of the reference numbers as
used in the drawings:
100: Conventional LCD device
101: Light source
102: Light guide plate
103: Diffusion sheet
104: Lower polarizer plate
105: Glass substrate
106: Color filter
107: Upper polarizer plate
108: Back light
201: Material 1
202: Material 2
203: Interface between the materials
204: Incident light
205: Reflected light
206: Transmitted light
While the exemplary preferred embodiments of the present invention
are described herein with particularity, those having normal skill
in the art will recognise various changes, modifications, additions
and applications other than those specifically mentioned herein
without departing from the spirit of this invention.
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